Radio frequency coil with subdivided iron core



1937. o. WOH LFAR TH ET AL 2,090,991

RADIO FREQUENCY con. WITH SUBDIVIDED IRON coma Filed May 7, 1935 ATTORNEY Patented Aug. 24, 1937 PATENT OFFICE RADIO FREQUENCY COIL WITHSUBDIVIDED IRON CORE Otto Wohlfarth, Berlin, and Alfred Agricola,Berlin-Tempelhof, Germany, assignors to Allgemeine ElektricitatzGesellschaft, Berlin, Germany Application May 7, 1935, Serial No. 20,160In Germany May 7, 1934 4 Claims.

The present invention relates to radio frequency inductance coils andmore particularly to such coils which are provided with powderedmagnetic material cores.

5 Radio frequency coils are known in the prior art which are furnishedwith subdivided ironcores, more particularly iron-cores made ofcompressed or molded iron powder. Coils of this kind, if the iron powderhas been chosen properl ly and also the insulation thereof, whencompared with air-cored coils, are of smaller dimensions for the sameloss. Another merit of coils of this kind consists in that they haveless leakage or stray. than air-cored coils.

Now, it has been found that with the applica- I tion of well known waysand means in making the iron-cores, i. e., extensive subdivision of theiron-core (grain size less than the ironlosses may be diminished to sucha point where 2 their share in the total losses becomes so small thatthe losses occasioned in the coil and which are a'fun'ction of thefrequency, especially capacitive losses and eddy-current losses,exercise a decisive influence upon the quality of the iron- 5 coredcoils. So far as the quality of the radio frequency coil, especially itsuse in tuning circuits covering a comparatively extensive frequencyrange or band is concerned, for instance, the range included inbroadcast waves, it is im- 30 portant that the frequency-dependentportion of the coil losses should be as low as possible in order thatthe ratio RmL will be stable and constant as far as. feasible for thefrequency range here concerned (where R dielectric loss 35 angle, Linductance, and w=frequency, 21r).

As has been borne out by experiments of which this invention is theoutcome, these losses are essentially affected by the shaping. and theselection of the material for the coils. Iron-cored 40 coils as known inthe prior art have mostly been of the kind with a cylindrical and a diskwinding. Both these types of winding, when employed in connection withiron-cored coils, involve the demerit that turns which have a1ter-. 45nating voltage are directly adjacent the iron core. Now, according tothis invention the winding is built up in such a fashion that the turnscarry-- ing alternating voltage are so'much farther removed from theiron core, the higher such alternating voltage applied to them, thewinding being at the same time so chosen that it will inherently be oflow capacitance. The losses by virtue of capacitive leakance will thenbe found n to be particularly low, and a coil thus designed exhibitssubstantially lower losses than the coils so far used in combinationwith iron-cores.

In the drawing, Fig. 1 illustrates one form of the invention wherein across wound coil is provided with a core; Fig. '2 illustrates diagram- 5matically the formation of the wire used to wind the coil; Fig. 3 is acurved sheet illustrating the characteristics of the coils; Fig. 4 is amodification of the coil structure shown in Fig. 1 and Figs. 5 and 6illustrate diagrammatically certain 10 preferred embodiments of theinvention.

The coil of the invention may be designed in the form of a cross-woundcoil the end of which located towards the iron-core is connected with afixed potential (grounded). The form of the coil in this scheme ispreferably so chosen that its width b will be severaltimes the length las shown in Fig. 1.

The cross-winding offers over the coil known in the art a chance forusing the coil unsupported, that is to say, without the use of a bobbinorcoil body. This insures the advantage that the inside crosssectionalarea of the coil wind ing is completely filled by the iron-core s(Fig. 1) with the result that the field of force set up in the interiorof the coil is homogeneous, and this means a further reduction ofthelosses.

According to another, object and idea of this invention, with a view toreducing the frequency v dependent coil losses still further, recourseis had to a multiple subdivision of the strands used for the winding, inother words, the wired the coil consists of a multiple-stranded litz.The latter, for instance, could be composed and built up in such a waythat several say, -three strands orlitzes each of which consists ofthree or more inter-stranded constituent or individual conductors, arein turn twisted or stranded together as shown in-Fig. 2, where al, a2,a3, designate thre'e'strands each of which is composed of three 40individual conductors, and these three strands are again twisted orstranded together. By such multiple stranding the losses due to skinellect are substantially diminished.

- In a form of coil as stated it is also possible to choose the diameterof the constituent conductors .of the strand extremely small so thatboth the eddy-current losses in the strand proper as well as thecapacitive losses of-the. coil will be reduced. At the same time, theincrease in the ohmic resistance of the coil due to the reduction in thecrOss-sectionalarea of the wire, and which is independent of thefrequency, has a favorable effect. As can be seen from Fig. 3, thesimultaneous increase in the ohmic resistance-which nection to give theiron core the shape of a bodyv plays a large part in the aggregate coillosses especially where low frequencies are dealt with,

leads-to a further fiatteningbf the loss curve.

The curve e which stands for a coil made of comparatively stout wiresand multiple-stranded litz, changes over to curve 1 when using a coil'made of slender wires and multiple subdivision of the strand, for thesame number of conducthroughout the frequency band becomes considerablymore constant.

Very favorable results were obtained with strands of 3 6 0.05 mm., inother words, strands in which the constituent wires or conductorsconsist of copper wire with a cross-section of only 0.05 mm., six ofwhich were twisted together so that there resulted three strands whichwere, in turn, stranded together.

For the purpose of a further reduction of the frequency-dependent lossesthe strand, according to the invention is lined with one or more coatsor layers of low-loss insulation material, such as low-loss paper, say,Japanese paper. In this manner spacing between the constituent spires ofthe coil is insured so that the field set up around the strand axis andwhich decreases rapidly with the distance will cut across the adjacentstrands only to a limited extent.

The reduction of the losses attainable by choosing a coil form ashereinbefore disclosed is still further decreasable by shaping the ironbody or core in a suitable way. In the iron-cored coils known to theearlier art, the said iron body mostly presents the form of an H or of aring. These forms inhere quite a number of disadvantages, inter alia,that the winding and thus live portions of the coils come to be placedat close proximity to the iron core, and this is conducive to additionalcoil losses. In order to reduce losses of this kind, according toanother basic idea of the invention the said iron-core is so formed anddisposed in reference to the coil that the live parts of the coil arespaced apart from the iron core. It is advantageous in this conofrotation.

Exemplified embodiments are shown in Figs. 4 to 6. Fig. 4 shows aniron-cored coil which consists of winding to which has the shape of across winding, and the iron core comprising These parts SI and s2 arecup? shaped bodies presenting rotational symmetry parts sl .and s2.

and whose bottom surfaces interengage. The supply-lead zl on the ironbody is grounded, while the opposite lead 22 is on voltage.

, As can be seen the edges of the cups areso bent away from the windingthat those parts of the winding which are at a higher potential arefarther removed from the iron body than the portions being at a lowerpotential. u

The fact that the iron body is cup shaped insures an adequately closedmagnetic path. But in order that the entire magnetic path may beimproved, the cups sl and s2, as shown in Fig. 5 may at the outermostedge 1" be again bent to-- wards one another. In this manner also thestray is diminished. If desired, as shown in Fig.

' shells are placed flatly against each other, then the bottom of thesame, and, if desired, also the edges of the same, could be madeelliptical instead of presenting'rotational symmetry so that by turningthe two cups sl and $2 in reference to each other it is possible toalter the inductance of thecoils, a fact, that may be of advantage inmatching or adapting coils.

We claim:

1. A radio frequency coil provided with a core composed of compressedfinely divided magnetic material, said core comprising a pair ofadjacent cooperating cup shaped members arranged with respect to eachother so as to form an enclosure for the coil, one of said members beingprovided with a central abutment upon which the coil is wound, saidabutment being placed on the side of the cup shaped member which isadjacent the other cup shaped member.

2. A'radio frequency coil provided with a core composed of compressedfinely divided magnetic material, said core comprising a pair ofadjacent elliptical cup shaped members mounted with respect tooneanother so as to form a substantial enclosure for the .coil, one ofsaid members being provided with a central abutment on the insideportion thereof, the coil being supported upon said abutment, said twomembers being displaceable relative to one another to thereby providemeans for varying the inductance value of the coil.

3. In an inductance device, a coil, a magnetic material core for thecoil, said core comprising two' shell halves, one of said shell halvesbeing provided with a pin, said coil being positioned around the pin,the other shell half having formed therein a recess for receiving thepin of the other shell ha1f,'said shell halves being constructed so thata substantial portion of the outer peripheries thereof are a greaterdistance respect to one another so as to form a substan-* tially closedmagnetic path, one of said members being provided with a centralabutment, said coil being. cross wound around said abutment, the radialdiameter of the coil being several times its length, leads forconnecting the coil to external circuits, one of said leads being on theouter periphery of the coil, the other thereof being connected to thecoil'at a point which is near the abutment, said last named lead beinggrounded when the core is connected to an external circuit,

the other lead being connected to a point of comparatively high voltagewhereby-the turns of the coil carrying the greater voltage are furthestaway from the abutment.

O'I'IO WOHLFARTH. ALFRED AGRICOLA.

